The present invention relates to measurements of low concentration of gases in the presence of other gases using the relative IR-absorbtion characteristics of the gas to be detected.
The presence of a particular gas can, for example, be determined by measuring the intensity of infrared radiation of particular wavelength before and after traversing a particular volume of gas, which may contain the particular gas, but at an unknown concentration. Most gases show definite lines due to rotational oscillatory absorbtion by the molecules. These absorbtion lines are particularly dominant in the range for wavelengths from 2 to 20 .mu.m and they are sufficiently distinctive to permit selective and even quantitative analysis of gases and regarding specific components thereof, even when present at rather low concentrations only. Measurements of this kind have become particularly important for determining the presence of contaminants within the framework of ecology and environmental protection. High specificity and selectivity requires utilization of highly monochromatic sources for radiation, and lasers have become of significant importance here.
A known method for measuring the concentration of gases is comprised of, first, determining the intensity of a particular laser beam after it has traversed the background gas under conditions in which the gas component of interest is positively absent, followed by (or concurring with) measurement of the intensities of radiation as traversing gas that may contain the component of interest. This type of measurement requires generally speaking, that the conditions are comparable under which reference and detective measurements are made. Thus, radiation intensity of the source, wavelength and other parameters, such as effectiveness of the optical devices, must remain constant and/or similar. Specifically, the absorbtion of the radiation by the background gas must remain the same. It was found that only relatively large concentrations could be detected at the desired degree of accuracy. Other methods required lowering of the pressure in order to obtain the desired sensitivity. Sampling of the gas becomes rather complicated and time-consuming under such circumstances.